First Exposed Planetary Core Discovered Allows Glimpse Inside Other Worlds

Artist's impression showing a Neptune-sized planet in the Neptunian Desert. It is extremely rare to find an object of this size and density so close to its star. (Image: University of Warwick/Mark Garlick)

The surviving core of a gas giant has been discovered orbiting a distant star by University of Warwick astronomers, offering an unprecedented glimpse into the interior of a planet. The planetary core, which is the same size as Neptune in our own solar system, is believed to be a gas giant that was either stripped of its gaseous atmosphere or that failed to form one in its early life.

The team from the University of Warwick’s Department of Physics reported the discovery on July 1 in the journal Nature, and it is thought to be the first time the exposed core of a planet has been observed. It offers the unique opportunity to peer inside the interior of a planet and learn about its composition. Located around a star much like our own approximately 730 light-years away, the core, named TOI 849 b, orbits so close to its host star that a year is a mere 18 hours and its surface temperature is around 1800°K.

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TOI 849 b was found in a survey of stars by NASA’s Transiting Exoplanet Survey Satellite (TESS) using the transit method: observing stars for the tell-tale dip in brightness that indicates that a planet has passed in front of them. It was located in the “Neptunian desert” — a term used by astronomers for a region close to stars where we rarely see planets of Neptune’s mass or larger. The transit signal was confirmed and refined using observations with 10 telescopes of the Warwick-led Next-Generation Transit Survey (NGTS), based at the European Southern Observatory’s Paranal Observatory in Chile.

The NGTS telescopes were specifically designed to detect the very shallow dips in brightness from exoplanets transits: in this case, only a tenth of 1 percent of the star’s brightness. The object was then analyzed using the HARPS instrument, on a program led by the University of Warwick, at the European Southern Observatory’s La Silla Observatory in Chile. This utilizes the Doppler effect to measure the mass of exoplanets by measuring their “wobble” — small movements toward and away from us that register as tiny shifts in the star’s spectrum of light.

The team determined that the object’s mass is 2-3 times higher than Neptune, but that it is also incredibly dense, with all the material that makes up that mass squashed into an object the same size. Lead author Dr. David Armstrong from the University of Warwick Department of Physics said:

There are two theories as to why we are seeing the planet’s core, rather than a typical gas giant. The first is that it was once similar to Jupiter but lost nearly all of its outer gas through a variety of methods. These could include tidal disruption, where the planet is ripped apart from orbiting too close to its star, or even a collision with another planet. Large-scale photoevaporation of the atmosphere could also play a role, but can’t account for all the gas that has been lost.

Alternatively, it could be a “failed” gas giant. The scientists believe that once the core of the gas giant formed then something could have gone wrong and it never formed an atmosphere. This could have occurred if there was a gap in the disc of dust that the planet formed from, or if it formed late and the disc ran out of material. Dr. Armstrong adds:

Provided by: University of Warwick [Note: Materials may be edited for content and length.]

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